In many explosive applications, a series of elongated, deep bore holes are provided in the material to be fragmented. Such bore holes are filled with an explosive material which is chosen on the basis of explosive characteristics and cost. In many instances, NCN and a variety of slurries are used as an explosive material because of their low cost. When these explosives are used, dynamite or cast primers are the commonly employed detonating devices. It has long been known that substantially more energy can be transmitted to the surrounding burden, if the explosive in the bore hole is detonated from a lower position. Consequently, substantial development work has been devoted to systems for detonating the explosive column at a lower position of a bore hole. The most widely used system for this purpose involves the use of an electric blasting cap. An electric blasting cap includes a housing having an explosive charge, which is capable of being detonated by an electrically heated resistance wire connected to two wires known as "leg wires". The leg wires extend from the blasting cap to a remotely located source of electrical current. When using this type of detonating system, the electrical blasting cap can be positioned below the surface of the explosive column within a bore hole with the leg wires extending from the blasting cap, through the explosive column and to any remote position. A current source applied across the leg wires fires the blasting cap and detonates the explosive column. This type of electrical system has proven quite useful for lower detonation of bore holes; however, certain disadvantages have become apparent. In blasting locations, electrically operated equipment is often used for various non-blasting work. Many times ground cables must carry electrical current for operation of such equipment. In addition, certain equipment generates electrical current for use by the equipment itself. Since the blasting sites are exposed to atmospheric condition, it is possible to experience lightning and static electricity conditions. It has also been found that when a number of leg wires are connected for simultaneous detonation of several bore holes, these conductive wires can form receiving antennas which will generate electrical currents when exposed to electromagnetic energy sources, such as radio transmitting antennas. All of these sources of stray electricity present a potential for inadvertent detonation of electrically actuated blasting caps after the caps are placed into bore holes. To overcome the possibility of inadvertent detonation by stray electricity at a blasting site, expensive precautions are required.
Because of the disadvantages of electrical blasting caps, it is somewhat common practice to detonate the upper portion of the explosive column in bore holes. In this manner, standard detonating cord can be used with a primer located at the upper portion of the explosive column. The disadvantages of electrical blasting caps are avoided; however, the additional explosive strength experienced with lower detonation of the explosive column is not obtained. To realize the benefit of lower detonation without using an electrical system, substantial effort has been devoted to development of a positive non-electrical system for detonating explosive columns at a position deep in a bore hole.
If a standard detonating cord, which does not present the basic disadvantages of an electrical system, is extended through an explosive column in a bore hole filled with NCN, slurry, dynamite or other explosive material, the explosive column is detonated from the top when the detonation wave in the cord reaches the explosive. This is due to the fact that the explosive wave of standard detonating cord is sufficiently strong to explode non-cap sensitive explosives in direct contact with the cord. For this reason, standard detonating cord can not be used for lower detonation of explosive columns in the confinement of bore holes.
To provide lower detonation of explosive columns, certain modifications have been made in detonating cord. The first proposed modification of detonating cord has been the development of a low energy detonating cord, often known as LEDC, which includes a small continuous lead tube filled with standard high explosive material with an approximate distribution or load of 1-2 grains per linear foot. This compares with a standard distribution of 15-40 grains per linear foot for "economy" cord and over 50 grains per linear foot for reinforced cord. By using this low explosive loading, in a flexible lead tube, sufficient detonating energy is created at a cut end of the tube for the purpose of initiating a blasting cap. As is known, a blasting cap is a standard component having a small primary, highly sensitive charge for converting a relatively small detonating force, such as created by a low energy cord, into a higher force for detonating a secondary charge. The secondary charge has sufficient bulk to detonate the explosive in a bore hole. This type of system requires a good physical contact between the lead tube and primary charge of the blasting cap. To assure a sound connection to the lead tube, the blasting cap is generally secured onto the detonating cord by a relatively expensive manufacturing operation performed at the manufacturer's plant. Consequently, the cap and cord must be purchased as a unit with the approximate length of cord being attached. If the cord length is not proper, it is not possible to splice the cord for changing its length. This caused difficulties in the field. This low energy type of system can be used for lower detonation if the proper connections are made at the initiating end and the blasting cap end. However, because of the sensitivity required to initiate this detonating cord, this system does not produce uniform results. If the blasting cap is not initiated after placed in a bore hole, it remains at the bottom of the bore hole in a dormant condition. As is well known, care must then be taken if the blasting cap is to be removed. Since the blasting cap includes a very sensitive primary charge an inadvertent blow can detonate the cap and any explosive adjacent thereto. Because of the uncertainty of ignition, the possibility of leaving a dormant blasting cap in the bore hole, and the high expense of this type of system, this system has not proven the solution to the problems outlined above, although the low energy wave of the detonating cord does allow it to pass through certain explosive material to the lower portion of an explosive column in a bore hole.
To overcome the disadvantage of requiring a demanding physical contact between the low energy detonating cord at both ends thereof and the cost contaminant thereto, a further type of lower energy detonating cord has been developed using the concept of a hollow plastic tube with the inner walls of the tube coated with a slight amount of high explosive material. This second type of low energy detonating cord is described in U.S. Pat. No. 3,590,739. Approximately 0.5 to 2.4 grains per linear foot of explosive material is used on the inside surface of a hollow plastic tube for detonating purposes. By using this structure, it is possible to extend the hollow detonating cord through an explosive column to a lower portion of a bore hole; however, since relatively low energy is created by the small amount of high explosive within the cord, this system again requires a sensitive blasting cap in the bore hole itself. By requiring a blasting cap in the lower portion of the bore hole, as required in the first low energy type of detonating cord, a very sensitive primary charge is used in connection with a secondary primer charge. Thus, if detonation does not occur, expensive precautions are necessary to remove the blasting cap at the bottom of a bore hole.
The two prior attempts to provide a blasting cord which can pass through the explosive charge of a bore hole for lower detonation thereof each have common disadvantages. They are both predicated upon the theory that a minute distribution or load of high explosive within the cord, less than about 4 grains per linear foot, is the proper procedure for preventing detonation of the charge as the cord is exploded through an explosive. The reduction of loading in the core of a blasting cord for reducing the probability of premature detonation in a bore hole causes substantial disadvantages. First, each low energy system requires a blasting cap in the bore hole. The low energy cord has insufficient usable energy for detonating a cast primer without a highly sensitive charge found in a blasting cap. Since these low energy cords use the concept of reduced available energy, detonation is less positive, especially in the variable ambient conditions within a bore hole. Another distinct disadvantage of low energy cord is the inherent inability to transmit a detonating wave to or from a standard detonating cord. Consequently, it is not practical to provide a standard detonating cord as a trunk line for direct connection to a low energy cord forming a down line of a bore hole. Thus, to initiate the low energy detonating cord, a strong positive initiating force must be exerted on the cord itself. This results in complications when multiple bore holes are to be shot simultaneously. In addition, low energy cords can not detonate from one cord to another. Thus, splicing of such cords is not practical. With all of the disadvantages inherent in using low energy detonating cord, relatively expensive blasting equipment is required and substantial expense is incurred by using such cord. Additional expenses are incurred to assure the safety of the site when an attempted detonation by low energy cord fails.